Among the widely-used instruments for measuring surface profiles are interferometers, which use the wave nature of light to map variations in surface height with high accuracy. Examples of common interferometers are given, for example, in Chapters 1, 2 and 3 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). Most of these conventional interferometric means cannot accommodate surface features with discontinuous height variations or surface roughness that exceed one-quarter of the wavelength of the source light. Such surface features result in interferometric phase ambiguities that are difficult or impossible to interpret. A further difficulty arises when the surface slope is so large that it becomes difficult to resolve the interference fringes.
As a consequence of the limited range of application for conventional interferometers, the prior art provides some alternative methods and means that seek to reduce the sensitivity of the measurement. One obvious method involves increasing the wavelength of the light through the use of unusual sources. An example method and apparatus is disclosed in the paper "Rough surface interferometry using a CO.sub.2 laser source," by C. R. Munnerlyn and M. Latta (Appl. Opt. 7(9) 1858-1859 (1968)). However, such methods are generally very expensive and cumbersome, since they involve specialized sources, optics and detectors.
Another prior-art approach to overcoming the limited range of conventional interferometers involves the use of multiple wavelengths, as originally described by R. Rene Benoit in the paper "Application des phenomenes d'interference a des determinations metrologiques," J. de Phys. 3(7), 57-68 (1898). A sequence of measurements at two or more wavelengths provides a much larger equivalent wavelength that overcomes some of the ambiguity problems of conventional single-wavelength interferometers. A method for applying this technique to surface metrology is disclosed in U.S. Pat. No. 4,355,899, issued to T. A. Nussmeier. However, these multiple wavelength techniques still do not function correctly when the surface slope is so large that it becomes difficult to resolve the interference fringes. In the prior art two-wavelength holographic method described by K. Haines and B. P. Hildebrand in an article entitled "Contour generation by wavefront reconstruction," Physics Letters, 19 (1), 10-11 (1965), the net result of two-wavelength reconstruction of the holographic image is the appearance of contour intervals of constructive interference. However, this method is difficult to use in practice.
A prior art method of profiling surfaces using scanning white light interferometry is disclosed in U.S. Pat. No. 4,340,306 issued to N. Balasubramanian. This patent describes a white-light interferometer that comprises a mechanically-scanned reference mirror, a two-dimensional detector array, and computer control. The disclosed prior art method involves scanning either the reference mirror or the object in discrete steps, measuring the fringe contrast for each point in the image for each scan position, and in this way determining for each surface point the position of maximum fringe contrast. The scan position for which the contrast is maximum is a measure of the relative height of a particular surface point. However, this prior art technique is very slow and does not work well with large, rough surfaces because of the difficulty in resolving the interference fringes.
The prior art also provides some alternative measurement geometries that seek to reduce the sensitivity of the measurement to surface roughness, surface slope and multiple reflections. A representative prior-art technique in this regard is described in U.S. Pat. No. 4,325,637 issued to R. C. Moore, in which a grazing incidence interferometer employs extreme angles of illumination in order to reduce the fringe density on the object surface. Another form of grazing incidence interferometer is described by J. Schwider, R. Burow, K. E. Elssner, J. Grzanna, and R. Spolaczyk in a paper entitled "Semiconductor wafer and technical flat planeness testing interferometer" (Appl. Opt. 25(7) 1117-1121 (1986)). However, the significant reduction in sensitivity in these prior art approaches requires a large illumination angle with respect to normal incidence. Such large angles create problems with proper illumination and imaging of the object. There may also be undesirable shadowing from surface features such as steps and channels.
Another form of prior art interferometer working at extreme angles of incidence is disclosed in U.S. Pat. No. 4,498,771 issued to G. Makosch. The disclosed apparatus uses a birefringent crystal, such as a Wollaston prism, and a system of mirrors to direct the light beam to the object. In FIG. 2a. of U.S. Pat. No. 4,498,771 there is depicted an embodiment that seeks to greatly increase the effective fringe spacing in the interference pattern by illuminating the object with two beams at two different angles, with this embodiment being described in columns 3 and 4 of the patent. However, the use of such a birefringent crystal limits the size of objects that may be observed to only a few centimeters at most. The apparatus also appears to be complicated and difficult to align.
Still another prior art approach to reducing the sensitivity of optical interferometers through alternative geometries is described by W. Jaerisch and G. Makosch in a paper entitled "Optical contour mapping of surfaces" (Applied Optics 12(7), 1552-1557 (1973)). This paper describes a prior art method that employs a diffractive optical element, in this case a diffraction grating, placed nearly in contact with the test surface. Illumination of the grating by a monochromatic plane wave generates an interference pattern between the beam components of two different diffraction orders. This pattern is reflected off of the object surface and is superimposed back onto the grating, resulting in a fringe pattern that resembles the surface contours of the object surface. These contours may be much larger than the wavelength of the source light. A similar prior art approach to creating desensitized fringes is described in a paper in a paper entitled "Common-path holographic interferometer for flatness testing" by P. Jacquot, X. Colonna de Lega and P. M. Boone (SPIE 2248, Optics for productivity in manufacturing, paper 18 (1994)). The instrument also works by the interaction of two diffraction orders of a diffractive optical element, however in this case the element is a holographic recording of a spherical wavefront.
Both the prior art method of Jaerisch and Makosch and the prior art method of Jacquot et al. require placing a diffractive element nearly in contact with the object surface. This is because in both methods a single diffractive element divides the source light into two beams which propagate in different directions and do not perfectly overlap on the object surface. The two beams are, therefore, not properly oriented for generating the desired interference effect, especially on rough surfaces. A further difficulty is that the two beams have an increasingly large optical path difference as the object is moved further away from the diffractive element; whereas the desired interference effect is most easily achieved when this optical path difference is small. The only way to avoid such problems in these prior-art optical profilers is to bring the object very close to the surface of the diffracting element. However, this very close working distance is undesirable and often impractical. Actual contact with the element can damage it severely, and a close working distance complicates parts handling and automation, particularly in industrial environments.
A further difficulty with the prior art in interferometric metrology arises when the object is partially transparent, since the resulting interference pattern is often a complex mixture of fringes created by reflections from both the front and back surfaces of the plate. In order to do meaningful metrology on such an object, the common practice is to either thinly coat the front surface with a high reflectivity material, or defeat the back-surface reflection by applying some kind of index-matching coating. These kinds of surface treatments are very undesirable for regular inspection and testing, particularly for process control in a production environment. In the copending, commonly-owned U.S. patent application entitled "Method and apparatus for profiling surfaces of transparent objects" (Ser. No. 08/153,146, filed Nov. 15, 1993 there is disclosed a method whereby the two surfaces may be mathematically isolated after two measurements, but this approach requires complicated apparatus and procedures.
As a consequence of the great difficulties in using interferometry for a variety of important applications, the prior art provides several optical profiling techniques that are not based on the wave nature of light. A representative example is moire fringe analysis. This prior art technique is described in detail in Chapter 16 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). The prior art moire method involves the projection and imaging of a ronchi ruling or like periodic structure, and is equivalent to geometric triangulation. A commercial product based on this principle is the Chek-flat, manufactured by Speedfam-Spitfire products group (Des Plaines, Ill. 60016). Although this product is capable of profiling rough surfaces, it is generally of very low accuracy when compared to optical interferometry, and does not work at all for specular surfaces.
Thus, the prior art techniques do not satisfactorily provide an apparatus and method for accurately profiling both rough and smooth surfaces using desensitized interference fringes at a useful working distance, or a method and means of profiling partially-transparent plane-parallel objects. These disadvantages of the prior art are overcome by the present invention.